What does enumerable mean?

An enumerable property is one that can be included in and visited during for..in loops (or a similar iteration of properties, like Object.keys()).

If a property isn't identified as enumerable, the loop will ignore that it's within the object.

var obj = { key: 'val' };

console.log('toString' in obj); // true
console.log(typeof obj.toString); // "function"

for (var key in obj)
    console.log(key); // "key"

A property is identified as enumerable or not by its own [[Enumerable]] attribute. You can view this as part of the property's descriptor:

var descriptor = Object.getOwnPropertyDescriptor({ bar: 1 }, 'bar');

console.log(descriptor.enumerable); // true
console.log(descriptor.value);      // 1

console.log(descriptor);
// { value: 1, writable: true, enumerable: true, configurable: true }

A for..in loop then iterates through the object's property names.

var foo = { bar: 1, baz: 2};

for (var prop in foo)
    console.log(prop); // outputs 'bar' and 'baz'

But, only evaluates its statement – console.log(prop); in this case – for those properties whose [[Enumerable]] attribute is true.

This condition is in place because objects have many more properties, especially from inheritance:

console.log(Object.getOwnPropertyNames(Object.prototype));
// ["constructor", "toString", "toLocaleString", "valueOf", "hasOwnProperty", "isPrototypeOf", "propertyIsEnumerable", /* etc. */]

Each of these properties still exists on the object:

console.log('constructor' in foo); // true
console.log('toString' in foo);    // true
// etc.

But, they're skipped by the for..in loop because they aren't enumerable.

var descriptor = Object.getOwnPropertyDescriptor(Object.prototype, 'constructor');

console.log(descriptor.enumerable); // false

If you create an object via myObj = {foo: 'bar'} or something thereabouts, all properties are enumerable. So the easier question to ask is, what's not enumerable? Certain objects have some non-enumerable properties, for example if you call Object.getOwnPropertyNames([]) (which returns an array of all properties, enumerable or not, on []), it will return ['length'], which includes the non-enumerable property of an array, 'length'.

You can make your own non-enumerable properties by calling Object.defineProperty:

var person = { age: 18 };
Object.defineProperty(person, 'name', { value: 'Joshua', enumerable: false });

person.name; // 'Joshua'
for (prop in person) {
  console.log(prop);
}; // 'age'

This example borrows heavily from Non-enumerable properties in JavaScript, but shows an object being enumerated over. Properties can either be or not be writable, configurable, or enumerable. John Resig discusses this in the scope of ECMAScript 5 Objects and Properties.

And, there's a Stack Overflow question about why you'd ever want to make properties non-enumerable.

It's a lot more boring than something that should be visualized.

There is literally an attribute on all properties called "enumerable." When it is set to false the for..in method will skip that property, pretend it doesn't exist.

There are a lot of properties on objects that have "enumerable" set to false, like "valueOf" and "hasOwnProperty," because it's presumed you don't want the JavaScript engine iterating over those.

You can create your own non-enumerable properties using the Object.defineProperty method:

  var car = {
    make: 'Honda',
    model: 'Civic',
    year: '2008',
    condition: 'bad',
    mileage: 36000
  };

  Object.defineProperty(car, 'mySecretAboutTheCar', {
    value: 'cat pee in back seat',
    enumerable: false
  });

Now, the fact that there is even a secret about the car is hidden. Of course they can still access the property directly and get the answer:

console.log(car.mySecretAboutTheCar); // prints 'cat pee in back seat'

But, they would have to know that the property exists first, because if they're trying to access it through for..in or Object.keys it will remain completely secret:

console.log(Object.keys(car)); //prints ['make', 'model', 'year', 'condition', 'mileage']

They should have just called it, "forInAble."

I will write one line definition of ENUMERABLE

Enumerable: Specifies whether the property can be returned in a for/in loop.

var obj = {};
Object.defineProperties(obj, {
    set1: {enumerable: true},
    set2: {enumerable: false},
});
Object.keys(obj); // ["set1"]
Object.getOwnPropertyNames(obj); // ["set1", "set2"]

If you're having difficulty visualising "what does it mean to be enumerable?" why not ask yourself, what does it mean to be nonenumerable?

I think of it a bit like this, a nonenumerable property exists but is partially hidden; meaning that nonenumerable is the weird one. Now you can imagine enumerable as what is left - the more natural property we're used to encountering since we discovered Objects. Consider

var o = {};
o['foo'] =  0;                               // enumerable, normal
Object.defineProperty(o, 'bar', {value: 1}); // nonenumerable, weird

Now in a for..in, imagine it like pseudocode

for property in o:
    if not property enumerable continue // skip non-enumerable, "bar"
    else do /* whatever */              // act upon enumerable, "foo"

where the body of the loop you typed in JavaScript is in the place of /* whatever */

What does enumerable mean?

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